When you use certificate authentication you have to set the uploadReadAheadSize in the IIS metabase to your max email size, the default is 48KB so nearly any attachment cannot be sent/forwarded.

With the following command you can change the value (in this case 10MB):
C:\Windows\System32\inetsrv\appcmd.exe set config -section:system.webServer/serverRuntime /uploadReadAheadSize:"10485760" /commit:apphost

Always backup your MySQL database with mysqldump but if you didn’t you can restore it from file system.

If your backup include the /var/lib/mysql directory, if so then you should be able to restore the database. Each database is contained in a separate subdirectory off /var/lib/mysql, so look for one named with the name of your database (WordPress or any other). To restore it, shutdown the mysql server first

sudo /etc/init.d/mysql stop

Copy the backup-ed directory into the same location on your new server, then restart mysql server

sudo /etc/init.d/mysql start

Note:- that its much better to take actual database dumps using the mysqldump command than to rely on this method. so for the future, you should schedule something like this: